FORMATION AND SPECIFICATION OF NEURONS DURING THE DEVELOPMENT OF THE LEECH CENTRAL-NERVOUS-SYSTEM

In the leech embryo, neurogenesis takes place within the context of a stereotyped cell lineage. The prospective germ layers are formed durin g the early cleavage divisions by the reorganization and segregation o f circumscribed domains within the cytoplasm of the fertilized egg. Th e majority of central neurons arise from the ectoderm, and central neu roblasts are distributed throughout both the length and width of each ectodermal hemisegment. Much of the segmental ganglion arises from med ial neuroblasts, but there are also lateral ectodermal neuroblasts and mesodermal neuroblasts that migrate into the nascent ganglion from pe ripheral sites of origin. Some of these migratory cells are committed to neurogenesis prior to reaching their central destination. In additi on, the leech embryo exhibits a secondary phase of neurogenesis that i s restricted to the two sex segment ganglia. Secondary neurogenesis re quires that a mitogenic or trophic signal be conveyed from the periphe rally located male sex organ to a particular set of centrally located neuroblasts, apparently via already differentiated central neurons tha t innervate the sex organ. The differential specification of neuronal phenotypes within the leech central nervous system occurs in multiple steps. Some aspects of a neuron's identity are already specified at th e time of its terminal cell division and would seem to involve the lin eal inheritance of developmental commitments made by one of the neuron 's progenitors. This lineage-based identity can then be modified by in teractions between the postmitotic neuron and other neurons or non-neu ronal target cells encountered during its terminal differentiation. (C ) 1995 John Wiley & Sons, Inc.